The present invention relates generally to a multilayer volume hologram (volume hologram seal) structure comprising a transparent pictorial hologram, which is pasted on a surface of a photograph or personal information area attached to ID cards, exam cards, passports, etc. for the purpose of ensuring security and a label for making such a multilayer volume hologram, and more particularly to a multilayer volume hologram which cannot possibly be fraudulently altered or otherwise falsified, and a label for making such a multilayer volume hologram structure.
In recent years, means for preventing fraudulent alternation or falsification of identity cards, etc. have been proposed, for instance, so as to certainly identify who carries an ID card as who is characterized by identifying data born thereon. Such falsification preventive techniques, for instance, are disclosed in JU-B 05-48215 and JP-A""s 05-201181 and 10-153944.
The techniques set forth in these publications make use of the fact that images carried on holograms have a function equivalent to that of inscriptions on conventional ID cards or the like. JP-A 05-201181 shows that when the protective film is stripped off for the purpose of falsification or the like, the surface relief hologram layer fractures so that it is impossible to reconstruct the hologram image, thereby preventing falsification as by making a photograph replacement. The surface relief hologram layer must be laminated in a cured or tackiness-free state so as to prevent the relief surface from being adversely affected.
On the other hand, a volume hologram comprises a photosensitive material wherein interference fringes are three-dimensionally recorded in its thickness direction. Recent developments of photosensitive resins and peripheral technologies make the mass fabrication of such volume hologram feasible. However, the volume holograms have tackiness and softness even when holograms are recorded, and so they are laminated on labels or the like while their tackiness is utilized. Thus, a grave problem with the volume holograms is that they are easy to falsify because protective filma are perfectly peeled off the volume holograms. To this end, techniques for preventing falsification by a volume hologram replacement have been proposed.
For instance, JP-A 10-153944 shows one counterfeit-proof technique using a volume hologram, wherein the peel strength between an application member and a hologram layer is larger than the breaking strength or tensile strength at 1% elongation of a hologram layer/surface protective layer multilayer structure, so that an attempt to peel the hologram layer off the application member causes a cohesive failure in the hologram layer or a discoloration of the hologram recorded therein, so that the hologram cannot be reused. However, since the volume hologram material is soft, the associated surface protective layer, too, must have the same physical properties. For instance, the surface protection of a volume hologram layer formed as a label or a multilayer hologram is still less than satisfactory.
JP-A 10-865665 shows a replacement-proof technique wherein in a multilayer structure comprising an application member/volume hologram layer/transparent protective sheet, adjacent layers are laminated together while an adhesive layer is interposed therebetween to provide an adhesion force distribution across it, so that an attempt to peel off the hologram layer results in a breakdown thereof. However, two adhesive agents must be used to provide such an adhesion force distribution. The operation involved does not only become complicated but also the use of different types of adhesive agents may be detected from the outside due to the refractive index difference.
JP-A 10-133552 shows that the adhesion between a protective film and a volume hologram layer is made weaker than the adhesion between an application member and the volume hologram layer, thereby generating a xe2x80x9crelease linexe2x80x9d in a direction perpendicular to the direction along which the volume hologram layer is peeled off, so that counterfeit-proofness is achievable. However, the peel line generated varies with the pulling direction and force for peeling off the hologram layer. Accordingly, no definite release pattern is obtained and so whether or not the hologram layer is falsified is still indefinite.
An object of the present invention is to provide a multilayer volume hologram which provides an improved protection means for a volume hologram and ensures the destruction of the volume hologram upon forcibly peeled off, so that counterfeit-proofness is achievable, and a label for multilayer volume hologram fabrication which enables a multilayer volume hologram to be efficiently fabricated.
According to the first aspect of the present invention, there is provided a multilayer volume hologram laminated on an application member and comprising an adhesive layer, a volume hologram layer and a surface protective film in this order, wherein the breaking strength of said hologram layer is larger than the peel strength between said surface protective layer and said hologram layer and smaller than the peel strength which said hologram layer is peeled off said application member and the breaking strength of said surface protective film.
Said surface protective film is releasable from said volume hologram layer, and said volume hologram layer is unreleasable from said application member, so that upon a forcible peeling of said volume hologram layer, it breaks down due to a cohesive failure.
According to the second aspect of the present invention, there is provided a multilayer volume hologram laminated on an application member and comprising an adhesive layer, a volume hologram layer and a surface protective film easily bondable to said hologram layer, wherein the breaking strength of said hologram layer is smaller than the peel strength between said surface protective film and said hologram layer and the peel strength between said surface protective film and said hologram layer is smaller than the peel strength with which said hologram layer is peeled off said application member and the breaking strength of said surface protective film.
Said surface protective film is laminated on said volume hologram layer with an adhesive layer interleaved therebetween.
A layer or layers capable of making adhesion strength between said hologram layer and a layer with said hologram layer laminated thereon partially different is/are provided on one or both surfaces of said volume hologram layer in a patterned manner, so that when said hologram layer together with said surface protective film is peeled off, said hologram layer is broken into a portion of said hologram layer corresponding to said layer capable of making said adhesion strength partially different and other portion of said hologram layer in a patterned manner.
Said volume hologram together with said surface protective film is unreleasable from said application member, so that upon a forced peeling of said volume hologram, said volume hologram breaks down due to a cohesive failure.
According to the third aspect of the present invention, there is provided a multilayer volume hologram comprising an application member and, in order from said application member, an adhesive layer, a volume hologram layer, a surface protective coating film and a substrate releasable from said surface protective coating film, wherein the breaking strength of a multilayer structure from which said substrate is released and which comprises the hologram layer and the surface protective coating film is smaller than the peel strength with which the multilayer structure comprising said hologram layer and said surface protective coating film is peeled off said application member.
A layer or layers capable of making adhesion strength between said hologram layer and a layer with said hologram layer laminated thereon partially different is/are provided on one or both surfaces of said volume hologram layer in a patterned manner, so that when said hologram layer together with said surface protective film is peeled off, said hologram layer is broken into a portion of said hologram layer corresponding to said layer capable of making said adhesion strength partially different and other portion of said hologram layer in a patterned manner.
Said volume hologram together with said surface protective film is unreleasable from said application member, so that upon a forced peeling of said volume hologram, said volume hologram breaks down due to a cohesive failure.
Preferably, said layer capable of making said adhesion strength between both layers partially different comprises a release agent in an amount of 1 part by weight to 20 parts by weight per 100 parts by weight of an adhesive resin binder.
Preferably, said adhesive layer comprises an adhesive layer/colored layer/adhesive layer, and said colored layer is selected from a colored adhesive layer, a printed layer, a light reflective layer and a colored resin film.
Preferably, said volume hologram layer has a breaking strength of 0.01 kgf/mm2 to 5 kgf/mm2 and a breaking elongation of 0.01% to 50%.
According to the fourth aspect of the present invention, there is provided a label for fabrication of any one of the aforesaid multilayer volume holograms, wherein instead of said application member, a release sheet is laminated on said adhesive layer.
The multilayer volume hologram of the present invention provides an improved protection means for a volume hologram and ensures the destruction of the volume hologram upon forcibly peeled off, so that counterfeit-proofness is achievable, and the label for multilayer volume hologram fabrication of the present invention enables such multilayer volume holograms to be efficiently fabricated.